The present invention relates to a charge transfer device (CTD) imaging array and, more particularly, to a novel focal point CTD array for providing a video output responsive to the amplitude and the phase of coherent light impingent upon each cell thereof.
There are many situations in which it is advantageous to be able to measure the optical path length between a pair of points (typically, between a fixed-location point and a point having a location to be determined) with an accuracy within a small fraction of one wavelength of the light of the measuring beam being utilized. In such applications, a coherent laser beam is normally used with a beam splitter, to obtain interference patterns between the light returning from the distant object-to-be-located, and a reference beam with known, and usually constant, phase and amplitude characteristics. By well known interferometry principles, changes in path length can be determined either as a function of position in the image plane, or, in the case of a moving object, as a function of time for a single point. In certain applications, improved performance can be obtained if a large number of path length measurements can be simultaneously taken. In Doppler velocimetry usage, the object (to which the distance is to be measured) is commonly a suspension of fine particles in a fluid or gaseous environment; the suspension is usually subject to a combination of incoherent and coherent motions. In this usage, it is possible to obtain an improved estimate of the coherent motion by averaging a number of independent views of the speckle pattern generated by the suspended particles. Since the phase of two points in a speckle pattern is uncorrelated if those two points are separated by more than the resolution limit of the optical system, a phase detecting imager can be used to advantage if the imager pixel spacing slightly exceeds the optical resolution of the apparatus. Each imager pixel will provide an independent estimate of the velocity; all of the individual pixel estimates can then be averaged to obtain an improved overall velocity estimate. In other applications, such as with Fourier optics, use of a phase-detecting imager can provide phase information at each array pixel within the aperture of an optical beam, so that the distribution of objects can be obtained, by Fourier transformation, from the amplitude and phase of the Fraunhofer diffraction pattern. It is therefore highly desirable to provide a focal point array in which coherent light amplitude and phase information can be detected, while maintaining an adequate signal to noise ratio and maintaining the imager output data rate at a manageable level. It is not possible to determine both the required phase information and the required amplitude information at each pixel of the array, with only a single measurement at that pixel.